Dual mode oxygen sensor

ABSTRACT

An oxygen sensor for measuring exhaust gases of an internal combustion engine is described. The sensor provides an output that has a high gain near the stoichiometric air-fuel ratio, and a lower gain in the lean and rich air-fuel ratio regions. Further, the sensor provides wide range air-fuel ratio signal to be used in engine feedback control.

TECHNICAL FIELD

The field of the invention relates to an exhaust gas oxygen sensor usedin engines of mobile vehicles to reduce emissions during a wide range ofoperating conditions.

BACKGROUND AND SUMMARY OF THE INVENTION

Engine exhaust systems utilize sensors to detect operating conditionsand adjust engine air-fuel ratio. One type of sensor used is a switchingtype heated exhaust gas oxygen sensor (HEGO). The HEGO sensor provides ahigh gain between measured oxygen concentration and voltage output. TheHEGO can provide an accurate indication of the stoichiometric point, butprovides air/fuel information over an extremely limited range (otherthan indicating lean or rich).

Another type of sensor used is a universal exhaust gas oxygen sensor(UEGO). The UEGO sensor can operate across a wide range of air-fuelratios, for example from 10:1 (rich) to pure air (lean). However, as aresult, the voltage to oxygen concentration has a lower gain.Furthermore, the UEGO sensor may not provide an indication ofstoichiometry as precise as the HEGO sensor, especially under widelyvarying temperature conditions.

The inventors herein have recognized that when an oxygen sensor is usedin a post catalyst position, the precise indication of stoichiometrygiven by the HEGO sensor provides advantageous results, but the limitedbandwidth degrades the capability of the control system to provide fastconvergence to desired operating conditions. Likewise, using an UEGOsensor can provide advantageous information when operating away fromstoichiometry, however, catalyst efficiency when operating aboutstoichiometry can degrade due to the imprecise measurement of thestoichiometric point.

One approach to try and correct for the UEGO sensor inaccuracies nearstoichiometry is described in U.S. Publication 2001/0052473. Here, thepower supply to the pump current is cut off, and a correction value isthen determined. However, the inventors herein have also recognized adisadvantage with such an approach. For example, the power supply can beturned off only in limited conditions, such as deceleration fuelshut-off, and thus an accurate reading of stoichiometry is onlyavailable under select conditions. Furthermore, the select conditionstypically do not include operation at stoichiometry under feedbackcontrol. As such, the measurement comes at an inappropriate time and isnot available when needed most. Further, errors due to variations intemperature can change depending on engine conditions, and as such evenif this correction is used, errors persist.

To overcome these disadvantages, and harness the respective advantagesof the above sensors, the following approach can be utilized.Specifically, in one aspect, a sensor is used that comprises: a firstreference cell having a reference voltage; a second pumping cell havinga pumping current, and a circuit configured to pump current in thepumping cell in a first direction to prevent the reference voltage fromincreasing higher than a first voltage limit; and to pump current in thepumping cell in a second direction to prevent the reference voltage fromdecreasing lower than a second voltage limit. In one example, when thecircuit pumps current in the pumping cell in the first direction toprevent the reference voltage from increasing higher than the firstvoltage limit, the circuit allows the reference voltage to decreaselower than the first voltage limit. Likewise, when the circuit pumpscurrent in the pumping cell in the second direction to prevent thereference voltage from decreasing lower than the second voltage limit,the circuit allows the reference voltage to increase higher than thesecond voltage limit.

In this way, the reference voltage can be driven by chemical reactionsto equilibrate and provide an accurate indication of stoichiometry,similar to a HEGO sensor. Likewise, outside of stoichiometry, thereference voltage is controlled in a one-sided fashion via positive andnegative pumping current at respective voltage limits to provide anindication of air-fuel ratio over a wide range.

An advantage of such operation is the ability to provide a signal thatis both accurate at stoichiometry and indicative of air-fuel ratio overa wider range. Such operation leads to more accurate feedback air-fuelratio control at stoichiometry with high gain sensing, while stillproviding air-fuel feedback information outside of stoichiometry, suchas for lean burn operation.

In another aspect, a method is provided for sensing an air-fuel ratio ofexhaust gasses from an engine using a sensor having a pumping cell and areference cell. The method comprises:

pumping current in the pumping cell during at least a first set ofoperating conditions;

reducing said pumping during at least a second set of operatingconditions;

providing a signal from said sensor during at least said first andsecond operating conditions; and

adjusting at least one of a fuel injection amount and an air amount intothe engine to maintain a desired air-fuel ratio based on said signalduring at least said first and second operating conditions.

As such, the method advantageously uses a sensor that both (1) pumpscurrent in the pumping cell during at least a first set of operatingconditions (such as to provide an indication of air-fuel ratio over awide range), and (2) reduces said pumping during at least a second setof operating conditions (such as about stoichiometry to allow chemicalequilibrium to drive a reference voltage). In this way, by using asignal from the sensor in both circumstances to provide feedbackair-fuel ratio control, accurate control can be obtained both aboutstoichiometry, and away from stoichiometry. Increased catalystefficiency and reduced emissions can also be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingexample embodiments in which the invention is used to advantage,referred to herein as the Description of Embodiment(s), in which likereference numbers indicate like features, with reference to the drawingswherein:

FIG. 1 is a block diagram of an engine utilizing an exhaust gas sensorto advantage;

FIGS. 2A and 2B are graphs showing example outputs of an HEGO sensor anda UEGO sensor, respectively;

FIGS. 3 and 4 show a schematic of dual cell sensor 140;

FIG. 5 is a graph illustrating experimental data;

FIGS. 6, 7, 8 and 9 are a high level flowchart and block diagrams ofcontrol routines;

FIG. 10 is state diagram of control logic; and

FIG. 11 is an alternative embodiment of control circuitry and an exhaustgas sensor.

DESCRIPTION OF EMBODIMENT(S)

Direct injection spark ignited internal combustion engine 10, comprisinga plurality of combustion chambers, is controlled by electronic enginecontroller 12 as shown in FIG. 1. Combustion chamber 30 of engine 10includes combustion chamber walls 32 with piston 36 positioned thereinand connected to crankshaft 40. In this particular example, piston 30includes a recess or bowl (not shown) to help in forming stratifiedcharges of air and fuel. Combustion chamber 30 is shown communicatingwith intake manifold 44 and exhaust manifold 48 via respective intakevalves 52 a and 52 b (not shown), and exhaust valves 54 a and 54 b (notshown). Fuel injector 66 is shown directly coupled to combustion chamber30 for delivering liquid fuel directly therein in proportion to thepulse width of signal fpw received from controller 12 via conventionalelectronic driver 68. Fuel is delivered to fuel injector 66 by aconventional high pressure fuel system (not shown) including a fueltank, fuel pumps, and a fuel rail.

Intake manifold 44 is shown communicating with throttle body 58 viathrottle plate 62. In this particular example, throttle plate 62 iscoupled to electric motor 94 so that the position of throttle plate 62is controlled by controller 12 via electric motor 94. This configurationis commonly referred to as electronic throttle control (ETC), which isalso utilized during idle speed control. In an alternative embodiment(not shown), which is well known to those skilled in the art, a bypassair passageway is arranged in parallel with throttle plate 62 to controlinducted airflow during idle speed control via a throttle control valvepositioned within the air passageway.

Exhaust gas oxygen sensor 76 is shown coupled to exhaust manifold 48upstream of emission control device 70. In this particular example,sensor 76 provides signal EGO, which indicates whether exhaust air-fuelratio is either lean of stoichiometry or rich of stoichiometry. SignalEGO is used to control engine air-fuel ratio as described in more detailbelow. In an alternative embodiment, sensor 76 provides signal UEGO tocontroller 12, which can convert signal UEGO into a relative air-fuelratio λ (air-fuel ratio relative to the stoichiometric air-fuel ratio,so that a value of 1 is the stoichiometric, with a value less than oneindicating rich, and a value greater than one indicating lean). SignalUEGO is used to advantage during feedback air-fuel ratio control in amanner to maintain average air-fuel ratio at a desired air-fuel ratio.Further, sensor 76 can be a sensor as described below in FIG. 3 or 4which provides a high gain signal at stoichiometry as well as a widerange air-fuel ratio signal.

Conventional distributorless ignition system 88 provides ignition sparkto combustion chamber 30 via spark plug 92 in response to spark advancesignal SA from controller 12.

Controller 12 causes combustion chamber 30 to operate in either ahomogeneous air-fuel ratio mode or a stratified air-fuel ratio mode bycontrolling injection timing. In the stratified mode, controller 12activates fuel injector 66 during the engine compression stroke so thatfuel is sprayed directly into the bowl of piston 36. Stratified air-fuelratio layers are thereby formed. The strata closest to the spark plugcontain a stoichiometric mixture or a mixture slightly rich ofstoichiometry, and subsequent strata contain progressively leanermixtures. During the homogeneous mode, controller 12 activates fuelinjector 66 during the intake stroke so that a substantially homogeneousair-fuel ratio mixture is formed when ignition power is supplied tospark plug 92 by ignition system 88. Controller 12 controls the amountof fuel delivered by fuel injector 66 so that the homogeneous air-fuelratio mixture in chamber 30 can be selected to be substantially at (ornear) stoichiometry, a value rich of stoichiometry, or a value lean ofstoichiometry. Operation substantially at (or near) stoichiometry refersto conventional closed loop oscillatory control about stoichiometry. Thestratified air-fuel ratio mixture will always be at a value lean ofstoichiometry, the exact air-fuel ratio being a function of the amountof fuel delivered to combustion chamber 30. An additional split mode ofoperation wherein additional fuel is injected during the exhaust strokewhile operating in the stratified mode is available. An additional splitmode of operation wherein additional fuel is injected during the intakestroke while operating in the stratified mode is also available, where acombined homogeneous and split mode is available.

Second emission control device 72 is shown positioned downstream ofdevice 70. Devices 70 and 72 can be various types of emission controldevices. As shown in FIG. 2, each device can contain multiple catalystbricks (70A, 70B, and so on; 72A, 72B, and so on). Alternatively, eachcan contain a single catalyst brick. In yet another example, the devicescan contain just one, two, or three bricks each. Additionally, varioustypes of catalytic converters can be used, such a three-way catalyticwashcoats. For example, three way catalysts that absorb NOx when engine10 is operating lean of stoichiometry can be used. In such catalysts,the absorbed NOx is subsequently reacted with rich exhaust gasconstituents (HC and CO, for example) and catalyzed during a NOx purgecycle when controller 12 causes engine 10 to operate in either a richmode or a near stoichiometric mode.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, anelectronic storage medium for executable programs and calibrationvalues, shown as read-only memory chip 106 in this particular example,random access memory 108, keep alive memory 110, and a conventional databus.

Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:measurement of inducted mass air flow (MAF) from mass air flow sensor100 coupled to throttle body 58; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a profile ignitionpickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft 40giving an indication of engine speed (RPM); throttle position TP fromthrottle position sensor 120; and absolute Manifold Pressure Signal MAPfrom sensor 122. Engine speed signal RPM is generated by controller 12from signal PIP in a conventional manner and manifold pressure signalMAP provides an indication of engine load.

In this particular example, temperatures Tcat1 and Tcat2 of devices 70and 72 are inferred from engine operation. In an alternate embodiment,temperature Tcat1 is provided by temperature sensor 124 and temperatureTcat2 is provided by temperature sensor 126.

Fuel system 130 is coupled to intake manifold 44 via tube 132. Fuelvapors (not shown) generated in fuel system 130 pass through tube 132and are controlled via purge valve 134. Purge valve 134 receives controlsignal PRG from controller 12.

In one example, exhaust sensor 140 is a second EGO type exhaust gasoxygen sensor that produces output signal (SIGNAL1). In an alternativeexample, sensor 140 can be a UEGO sensor. Finally, in still anotherexample, sensor 140 is a sensor as described below with regard to FIG. 3or 4. For example, if the sensor of FIG. 3 is used, then two outputsignals may be provided to controller 12. Alternatively, signal 1 can beV_(out) from FIGS. 6 and 7, for example.

While FIG. 1 shows a direct injection engine, a port fuel injectionengine, where fuel is injected through a fuel injector in intakemanifold 44, can also be used. Engine 10 can be operated homogeneouslysubstantially at stoichiometry, rich of stoichiometry, or lean ofstoichiometry.

Those skilled in the art will recognize, in view of this disclosure,that the methods described below can be used to advantage with eitherport fuel injected or directly injected engines.

Note also, that in one example, devices 70 and 72 are three-waycatalysts.

Referring now to FIGS. 2A and 2B, a description is given for two typesof exhaust gas oxygen sensors that are used in the automotive industry;a switching (HEGO) sensor and a wide range (UEGO) sensor. The HEGOsensor provides a high gain and operates close to stoichiometry butprovides air/fuel information over a limited range. The UEGO sensoroperates across a wide range of air fuel ratios; typically from 10:1 toair, but as a result has a lower gain. And, the UEGO sensor does notprovide as precise an indication of stoichiometry as the HEGO sensor.Dual cell UEGO sensors measure the air fuel ratio by measuring theoxygen pumping current required to maintain a stoichiometric air fuelratio in a cavity inside the sensor as measured by an internal HEGO likereference circuit. FIG. 2A shows an example HEGO sensor response, andFIG. 2B shows an example UEGO sensor response. As discussed above, whileeach of these sensors has its advantages, each also has itsdisadvantages.

For example, when a sensor is used in a post catalyst position, theprecise indication of stoichiometry given by the HEGO sensor is useful,but the limited bandwidth limits the capability of the control system.Further, the limited range of accurate information also limits use awayfrom stoichiometric operation (such as during feedback lean air-fuelratio, or rich air-fuel ratio, control).

To overcome at least some of these disadvantages, in one example, amethod for controlling a sensor having at least two cells to provide aHEGO-like signal at stoichiometry and a UEGO-like signal at air fuelratios away from stoichiometry is described. For example, as describedin more detail below, by turning the pumping current off atstoichiometry and providing a signal that blends the output of both thereference and pumping circuit, such a result is possible.

Referring now to FIG. 3, a portion of sensor 140 is described in moredetail. Specifically, oxygen pumping cell 302 is shown with a solidelectrolyte material 310, such as zirconia. Pump electrode pair 332 isshown mounted to solid electrolyte material 310. The electrodes can beof various types, such as platinum, and provide a pumping current 320,creating a flow of oxygen molecules depending on the direction ofcurrent.

Reference cell 304 is formed via substrate 316 comprised also of a solidelectrolyte material such as zirconia (ZrO2) having electrode pair 330.Air is introduced via hole (or reference cavity) 318, and an electrodepair 330 is also shown. A porous diffusion passage 314 is coupledbetween substrate 310 and 316, creating the hollow reference chamber (ordetection cavity) 312. A reference voltage 322 is provided via theelectrode pair 330. Note that a heater 340 for heating the sensor can beadded, if desired.

As shown, the detection cavity is exposed to the exhaust gasses via thediffusion passage. In this system, in a first range, the a dual cellsensor measures the air-fuel ratio via the oxygen pumping currentrequired to maintain a stoichiometric air fuel ratio in the cavity 312inside the sensor as measured by an internal reference voltage 322.Specifically, the sensing cell reacts to the air-fuel ratio of thedetection cavity and is used to control the pumping cell that will thenpump oxygen in or out of the detection cavity. By controlling thepumping cell such that the reference cell maintains a constant voltage(typically 0.45) the pumping current will then correlate to the air fuelratio of the exhaust gasses. For example, an interface circuit thatmeasures the pumping current and creates a signal that can be measuredby a powertrain control module (see FIG. 1) can be used.

However, rather than using the pumping cell to hold reference cell to afixed voltage under all conditions, the pumping cell is used to keep thereference cell voltage from exceeding pre-determined upper and lowerlimits in a one-sided control fashion. When the reference cell is withinthese limits, the reference voltage is used as an accurate, high gainair/fuel indication. At the limits, when the pumping is active, thepumping current is used to indicate the air fuel ratio across asignificantly wider range.

Thus, rather than controlling the reference cell to a fixed voltage andrelying on the measurement of the pumping current to indicate air fuelratio under all conditions, one of the example methods described hereinallows the reference cell to float within some range and uses both thereference cell voltage and pumping current (when active) to indicate theair-fuel ratio. This provides both the high accuracy and high gain ofthe HEGO sensor at stoichiometry and the wide range capability of theUEGO sensor into a single output signal.

One reason for this increased accuracy is that when the pumping cell isnot active, i.e., the reference voltage is within selected limits, theoutput signal is driven my chemical equilibrium reactions, and thus hasreduced sensitivity to external factors, such as exhaust temperature,etc. However, when the pumping cell is active to maintain the referencevoltage at either the upper (lean) limit or the lower (rich) limit, anindication of air-fuel ratio over a wide range can be achieved.

FIG. 4 shows sensor 140 having an example circuit for processing thereference voltage and controlling pumping current.

The circuit is shown coupled to electrodes 330 and 332 and sensor 140.The output of the internal electrode of 332 is coupled to ground 434.The outer electrode of 332 is coupled to resister 414 and generatesvoltage output (V_(p)) across it. In addition, operational amplifier 410is shown with the negative terminal coupled to the inner electrode 330.The positive terminal of amplifier 410 is coupled to ground 418 via the0.7 volt source. The 0.7 volt source represents an example upper voltagelimit, which is set as the lean limit in this example. However, variousother voltage levels can also be used, including a variable voltagelevel changing based on operating conditions. The output of operationalamplifier 410 is coupled through a diode 412 to resistor 414.

Continuing with the circuit shown in FIG. 4, operational amplifier 420is also shown with the negative terminal coupled to the inner electrodeof 330. The negative electrode of operational amplifier 420 is also readout through the voltage (V_(r)) 430, which is then coupled to ground432. The voltage output (V_(r)) 430 represents a reading of thereference voltage for the reference cell of sensor 140. Operationalamplifier 420 is also shown with the positive terminal coupled to ground426 through the 0.2 voltage source 424. The 0.2 voltage sourcerepresents a rich limit value, and as described above with regard to the0.7 voltage limit value of 416, this is just one example value for thislower limit. The output of 420 is shown coupled to 422 pumped voltageoutput (V_(p)). Note that the diode 412 and the diode 420 are showncoupled in opposite directions. This illustrates how the one-sidedcontrol of voltage at the limit value is accomplished with a relativelysimple circuit.

Various other circuits can be used to perform the desired acts andoperations, or add additional signal conditioning and modification.

This shows an example outline of the proposed control circuit. When thereference cell voltage is between the 0.2 volt and 0.7 volt limits,neither of the control circuits will supply pumping current (or willsupply a reduced current), and the reference cell voltage (V_(r)) can beused as the sensor output. When the reference cell voltage reaches 0.2volts (indicating a slightly rich mixture), the lower amplifier will beable to provide the pumping current necessary pump oxygen into thedetection cavity to hold the reference cell at 0.2 volts. This is anexample of one-sided control since the circuit will not prevent thevoltage from increasing past 0.2 volts. The air fuel ratio measurementwould then be derived from the pumping current output (V_(p)).Conversely when the reference voltage reaches the 0.7 volt limit, theupper amplifier will generate the required pumping current to maintainthe reference voltage at 0.7 volts. However, this is also an example ofone-sided control (although in the opposite direction) since the circuitwill not prevent the voltage from decreasing below 0.7 volts.

As shown above, the circuit has two separate outputs, V_(p) and V_(r).An alternate implementation would mix the two voltages to provide asingle output similar to the example graphed in FIG. 5 (see FIGS. 6 and7, for example).

It can be seen from FIG. 5 that the output from the sensor, whencontrolled according to this invention, provides both the accurate, highgain signal near stoichiometry and the wide range air-fuel ratio output(such as that normally obtained from the UEGO sensor).

In this way, it is possible to control a dual cell UEGO sensor toprovide a HEGO like signal at stoichiometry, while at the same time toprovide a UEGO like signal at air-fuel ratios away from stoichiometry byturning the pumping current off at stoichiometry and providing a signalthat blends the output of both the reference and pumping circuit.

As such, in one example, only a single signal is advantageously used toprovide both high resolution and wide range air-fuel signal. Such anapproach is advantageous relative to a system that, for example, usestwo signals to provide such information. For example, the single signalapproach requires fewer wires between the sensor and controller, as wellas fewer A/D converters and less potential degradation.

The circuits described above with regard to FIG. 4 show one aspect ofthe signal and sensor 140. However, other approaches can be used, suchas digital signal processing. To illustrate the general approach used,regardless of whether digital or analog circuits are used in theimplementation, FIG. 6 shows a block diagram of example signalconditioning that can be used. Specifically, the voltage referencesignal 610 is shown leading to a summation 612 and 614. At summation612, the signal V_(r) is compared to a lower lean voltage reference of0.2 (block 618). This difference is sent to a high gain amplifier (k) at620. Likewise, the reference voltage is also compared to an upper richvoltage reference of 0.7 (621), the difference of which is also sent toan amplifier of gain (k) at 622. The output of gain 620 is fed to thesaturation block 624 which limits the pumping current at positivevalues. Similarly, the output of gain 622 is fed to saturation block 626which limits pumping current to negative values. The sum of the outputsof block 624 and 626 are fed to a summation 630, the output of which isthe pumping current (i_(p)). Then, if desired, the pumping current andvoltage reference are sent to block 632 to be modified to produce asingle output (V_(out)). Alternatively, or in addition, the pumpingcurrent can be output at block 636.

This block diagram of FIG. 6 has three modes which are determined basedon the reference voltage (V_(r)).

If the reference cell output is less than 0.2, the upper portion of thecontrol circuit will generate a positive pumping current and control thereference cell to 0.2. The saturation block 624 blocks the output of thelower portion of the circuit. Conversely when the reference cell voltagereaches 0.7 volts, the lower portion of the circuit will generatenegative pumping currents to control the reference cell voltage. Whenthe reference cell voltage is between 0.2 and 0.7, the outputs from bothof the controllers will be blocked by their saturation blocks and nopumping current will flow; yet, the reference voltage will be driven bythe oxygen concentration of the measurement exhaust gas.

As indicated above, two output signals can be used (V_(p) and V_(r)) toprovide a high gain output signal (V_(r)) and a wide range output signal(V_(p)). However, in an alternative embodiment, these two separateoutputs can be combined into a single output signal as indicated byblock 632 in FIG. 6, which is now described with regard to FIG. 7. Theblock diagram of FIG. 7 shows how the two inputs (pumping current 710and reference voltage 712) are combined to a single output voltage(724). Specifically, the pumping current is inverted in block 714 bymultiplying it by negative one from block 711. Then, this signal is sentto a gain (k₂) in block 716 to produce a pumping voltage (V_(p)).Likewise, the reference voltage is sent to a gain at block 718 of k₃which is indicated as V_(re). The outputs of blocks 716 and 718 are thensummed at summation 720, along with an opposite voltage (B from block722). The output of the summation is then sent to the output at block724.

Thus, while the reference cell voltage and a voltage that correlates tothe pumping current could be output independently, in this example theabove circuit combines them into a single output to reduce the number ofinputs circuits required to read the sensor output.

Referring now to FIG. 8, a routine is described for adjusting fuelinjection using feedback control in the information from the exhaust gassensors 76 and 140. First, in step 810, the routine selects a desiredupstream air-fuel ratio (λ_(1d)). Next, in step 812, the routine selectsa desired downstream air-fuel ratio (λ_(2d)). Next, in step 814, theroutine reads the upstream air-fuel value from sensor 76, and in step816, the routine reads the downstream air-fuel value from a sensor 140as described above herein. Then, in step 818, the routine determineswhether stoichiometric feedback control is active. In other words, theroutine determines whether closed-loop feedback control aboutstoichiometric air-fuel ratio is selected. When the answer to step 818is no, the routine continues to step 820. In step 820, the routinedetermines whether non-stoichiometric feedback air-fuel control isselected. For example, the routine can determine whether rich air-fuelratio control or lean air-fuel ratio control is selected at a desiredrich or lean air-fuel ratio. When the answer to step 820 is no, theroutine goes to step 822 and carries out open loop air-fuel ratiocontrol independent of sensors 76 and 140.

Alternatively, when the answer to step 818 is yes, the routine continuesto step 824 to select a first set of control gains for PID controllersused to feedback control both the downstream and upstream air-fuel ratioto the desired values. Alternatively, when the answer to step 820 isyes, the routine continues to step 826 to select a second set of gainsfor the PID controllers. In other words, a first set of control gains isused for the high gain sensor output of sensor 140 in the stoichiometricregion, whereas a second set of control gains is used for the wide rangesignal output from sensor 140 away from stoichiometry.

Then, in step 828, the routine calculates a desired fuel injectionamount based on errors between the desired air-fuel ratios and thevalues from sensors 76 and 140, respectively. This fuel injectioncalculation is determined using the selected gains for the currentoperating conditions in a proportional interval derivative (PID)feedback control system.

In this way, it is possible to advantageously utilize the multi-purposesignal output from sensor 140, as shown by FIG. 3 or 4, for example.

Note that while the above approach modifies the desired fuel injectionamount to control the air fuel ratio, alternative approaches can beused. For example, when using an electronically controlled throttleplate (e.g., via an electric motor controlled by the controller) it canbe desirable to modify the air flow to control air fuel ratio. In otherwords, rather than scheduling an air flow as a function of the driverdemand (and setting fuel to achieve the desired air fuel ratio), one canschedule a fuel flow based on the driver demand (or other engine torquerequest) and calculate the required air to provide a desired air-fuelratio. Further, feedback can be used to modify the desired airflow toobtain the target air fuel ratio using information from the sensordescribed above. Such an approach can provide accurate air-fuel ratiocontrol with less torque disturbances. Still another alternativeapproach would be to modify both air and fuel based on the sensor.

Referring now to FIG. 9, a routine for determining degradation of sensor140 is described. First, in step 910, the routine determines whetherdegradation detection is enabled. For example, degradation detection ofsensor 140 can be enabled based on a combination or one of thefollowing: whether the time since engine start is greater than apredetermined value, whether engine cooling temperature is greater thana determined value, whether the engine has traversed a variety ofair-fuel ratios across the range of sensor 140, or various otherconditions. When the answer to step 910 is yes, the routine continues tostep 912. In step 912, the routine reads the sensor output of sensor 140(V_(out)). Then, in step 914, the routine compares the read value to theexpected value as would be determined from stored information such asthat illustrated in FIG. 5. Then, in step 916, the routine determineswhether error between the expected and read threshold value is greaterthan a threshold value. Note that, rather than a single comparison, amultitude of comparisons over a variety of air-fuel ratios representingthe range of air-fuel ratios over which sensor 140 can be used. When theanswer to step 916 is yes, the routine continues to step 918 to indicatethe degradation, such as via an indicator lamp to the driver.

Note that, in the event the pumping circuit degrades (but the referencecell is still operating appropriately, failed, the sensor can stillprovide a limited output similar to a HEGO, which can be used duringdefault operation. If, however, the pumping call or circuit degrades,default operation is selected to be carried out with open-loop fuelcontrol.

In this way, it is possible to determine degradation of a sensor havingan output that has both a high gain near stoichiometry, and a wide rangeair-fuel ratio output, as well as schedule default operation.

Referring now to FIG. 10, a state diagram of control logic for use witha sensor of the type as sensor 140 and/or 1140 is shown. Specifically,the figures shows how engine modes, in this example air-fuel ratiocontrol modes, may be changed depending on the output of the sensor.Mode 00 includes open loop (and/or closed loop) stoichiometric feedbackcontrol, which can include where the engine air-fuel ratio is rampeduntil a “switch” of the sensor is detected, at which point the air-fuelratio is adjusted to jump and ramp in the opposite direction. In thisway, oscillation about stoichiometry can be achieved. Alternatively, theair-fuel ratio can be adjusted independent of the sensor output.

Mode 10 includes lean closed loop feedback air-fuel ratio control at adesired lean air-fuel ratio value, where feedback is obtained fromsensor 140 providing an indication of the degree of leanness in theexhaust gas. The control transitions from Mode 00 to Mode 10 when thesensor voltage (Vs) is greater than a lean limit value (0.7 volts inthis example). Further, the control transitions from Mode 10 to Mode 00with the pump cell current (Ip) is less than a threshold value(designated Upper Limit in this example).

Mode 01 includes rich closed loop feedback air-fuel ratio control at adesired rich air-fuel ratio value, where feedback is obtained fromsensor 140 providing an indication of the degree of richness in theexhaust gas. The control transitions from Mode 00 to Mode 01 when thesensor voltage (Vs) is less than a rich limit value (0.2 volts in thisexample). Further, the control transitions from Mode 01 to Mode 00 withthe pump cell current (Ip) is greater than a threshold value (designatedLower Limit in this example).

In this alternative method for the sensor 1140 for detecting air-fuelratios, Mode 00 may be an open-loop, “HEGO-like” mode, using only thevoltage reference cell to detect air-fuel ratios near stoichiometry,with greater sensitivity. And Mode 10 can use the sensor in a leanfeedback manner (“UEGO-like”, with the pumping cell enabled, to detectair-fuel ratios away from stoichiometry.

FIG. 10 thus illustrates one of several possible sequences of operation.When control logic determines that the sensor is ready for operation(such as based on a time since engine start, or based on the sensorresponse), the sensor goes into Mode 00. The output from the circuit canbe either a fixed output or the reference cell voltage (Vs) passedthrough an amplifier and connected to the output terminal.

If the air-fuel mixture becomes lean, Vs quickly exceeds 0.7 volts, thecomparator labeled “Vs > Lean Limit” sends an output to the dual modecontrol logic. The pumping cell is then enabled, Vs is held at 0.45volts, and the pumping cell current (Ip), as measured by the senseresistor (Rs), is passed through an amplifier(s) to the output terminal.The sensor is now in the Lean “UEGO-like” state (Mode 01).

If the air-fuel mixture then becomes rich enough, Ip will decrease to avalue that causes the comparator “Ip < Upper Limit” to send an output tothe dual mode control logic. Ip is disabled and the sensor is in Mode 00again.

If the mixture continues to become richer, Vs decreases below 0.2 volts,the comparator “Vs < Rich Limit” sends an output to the dual modecontrol logic. Ip is enabled, Vs is held at 0.45 volts, and Ip, measuredacross Rs, is passed through amplifier(s) to the output terminal. Thesensor is now in the Rich “UEGO-like” state (Mode 01).

If the mixture then becomes lean enough, Ip will increase to a valuethat causes comparator “Ip > Lower Limit” to send an output to the dualmode control logic. Ip is disabled and the sensor is in Mode 00 again.

The paragraphs above show the state transitions that can occur, asillustrated by the attached logic state diagram. The specific values forIp Lower Limit or Ip Upper Limit can be selected as desired based on theapplication. The optimum values can be determined empirically.

Referring now to FIG. 11, an alternative sensor and circuitconfiguration is shown, optionally for use with the logic of FIG. 10. Inthis alternative configuration, additional components and circuitry havebeen added to provide additional functionality.

Specifically, this alternative embodiment shows sensor 1140 with a pumpcurrent cell 1102 and a voltage reference cell 1104. The figure alsoidentifies the direction of exhaust gas flow at 1106 and the porousdiffusion passage(s) at 1108. Gas detection cavity 1110 and O2 referenceelectrode 1112 are also indicated on the sensor. Further, a ceramicheater 1114 is shown for heating the sensor 1140.

The output of the circuits is shown coupled to various amplifiers andcircuitry. Specifically, the output voltage to the heater 1114 is fed toa sample and hold amplifier 1116, receiving command controls from aheater voltage supply circuit 1118 (which can be controlled via controllogic to be selectively activated based on engine and exhaust gasoperating conditions). In this example, control signals are generatedfrom the control logic at 1150, which may includes the logic of FIG. 10,as well as other control logic. A battery overvoltage amplifier 1119 isalso shown coupled to the heater supply 1120, to disable the voltagesent to heater 1114.

Continuing with FIG. 11, a current supply 1122 is shown coupled to thereference electrodes 1112. Further, the reference cell is also coupledto buffer amplifier 1124 and feedback amplifier and Loop filter 1126,along with analog switch 1128 and amplifier 1130 to providebidirectional pump current (Ip).

Another set of amplifiers, including a chopper circuit and resistors,1132, is shown for measuring the pump current via resistor Rs at 1134.Comparators 1136, 1137, 1138, and 1139 are also shown for providingcontrol input signal to block 1150. Also, control logic for the choppercircuit, and test logic, may be contained in block 1142. Both block 1150and 1142 receive input from a clock generator (1144).

In general, the above circuitry can be referred to as: heater control,feedback circuitry for stable operation of the sensor, Ip driver, Icpfor controlled leakage into Vs, chopper amplifier to minimize outputerrors and drift, and Z test to determine cell readiness for operation.

This concludes the description of the invention. The reading of it bythose skilled in the art would bring to mind many alterations andmodifications without departing from the spirit and the scope of theinvention. Accordingly, it is intended that the scope of the inventionbe defined by the following claims. Further, the following claimsparticularly point out certain combinations and subcombinations regardedas novel and nonobvious. These claims may refer to “an” element or “afirst” element or the equivalent thereof. Such claims should beunderstood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements. Othercombinations and subcombinations of features, functions, elements,and/or properties may be claimed through amendment of the present claimsor through presentation of new claims in this or a related application.Such claims, whether broader, narrower, equal, or different in scope tothe original claims, also are regarded as included within the subjectmatter of the present disclosure.

1. A system comprising: an oxygen sensor having a pumping cell and areference cell and producing an output voltage, the reference cellincluding an air-filled chamber and producing a reference voltage;electronics configured to measure the reference voltage, and furtherconfigured to turn off a current to the pumping cell to allow thereference voltage to float within a voltage range, and to apply thecurrent to the pumping cell if the reference voltage is outside thevoltage range; the electronics including first and second amplifiersarranged in parallel and configured to supply the current to the pumpingcell, the first amplifier applying the current to the pumping cell in afirst direction and the second amplifier applying current to the pumpingcell opposite the first direction; the output voltage based on thereference voltage and on the current to the pumping cell.
 2. The systemof claim 1, the electronics further comprising a first and secondfeedback loop, the first feedback loop including the first amplifier anda first diode; and the second feedback loop including the secondamplifier and a second diode.
 3. The system of claim 2, wherein saidcurrent flows through the first diode in a first direction and throughthe second diode opposite the first direction.
 4. The system of claim 1,wherein the electronics includes a storage medium storing digital code.5. The system of claim 1, wherein the electronics maintains thereference voltage at limits of the voltage range by increasing saidcurrent in the first direction if the reference voltage is at a firstlimit, and increasing said current opposite the first direction if thereference voltage is at a second limit.
 6. A system comprising: anoxygen sensor having a pumping cell and a reference cell and producingan output voltage, the reference cell including an air-filled chamberand producing a reference voltage; electronics configured to measure thereference voltage, to apply a current to the pumping cell in a firstdirection to prevent the reference voltage from increasing above a firstvoltage limit, and to apply a current to the pumping cell opposite thefirst direction to prevent the reference voltage from decreasing below asecond voltage limit; the electronics including first and secondamplifiers arranged in parallel to supply the current to the pumpingcell, the first amplifier applying the current to the pumping cell in afirst direction and the second amplifier applying the current to thepumping cell opposite the first direction; the output voltage based onthe reference voltage and on the current to the pumping cell.
 7. Thesensor of claim 6 wherein the electronics is further configured to applycurrent to the pumping cell to prevent the reference voltage fromincreasing above the first voltage limit, but to allow the referencevoltage to decrease below the first voltage limit without applying thecurrent to the pumping cell.
 8. The sensor of claim 7 wherein theelectronics is further configured to apply current to the pumping cellto prevent the reference voltage from decreasing below the secondvoltage limit, but to allow the reference voltage to increase above thesecond voltage limit without applying the current to the pumping cell.9. The sensor of claim 8 wherein the electronics is further configuredto output a signal responsive to both the current in the pumping celland the reference voltage.